Vehicle transmissions are well known and include automatic, manual and semi-automatic types. A trend in the industry has been to make vehicles more efficient to reduce operation costs and pollution. Accordingly, hybrid vehicles having two separate power sources have become increasingly popular. A typical hybrid includes an internal combustion engine as one of its power sources, while the second power source may be an electric motor or hydrostatic transmission system. In either arrangement, the primary and secondary power sources are typically connected to the vehicles drivetrain, each being capable of providing power to the wheels of the vehicle either separately or in tandem.
One type of hybrid vehicle system includes an internal combustion engine, a reversible, variable displacement hydraulic motor/pump which can be driven by the internal combustion engine, an energy accumulator supplied by said motor/pump, and at least one reversible hydraulic driving motor for propelling the vehicle that is supplied by the hydraulic accumulator and/or by said motor/pump operating as a pump and being driven by the engine. A transmission unit connects the engine with the reversible, variable displacement hydraulic motor/pump during a first mode of operation (city mode) and connects the engine to the vehicle drive wheel during a second mode of operation (highway mode).
In a city driving mode, the hydraulic accumulator is recharged intermittently and automatically by the reversible hydraulic motor/pump operating as a pump driven by the engine.
Between two charging stages, the internal combustion engine can be shut down to conserve fuel. When the pressure of the accumulator falls below a set value, the engine is started by the hydraulic motor/pump operating as a motor that is supplied pressurized fluid from the accumulator. The use of such accumulators and reversible hydraulic, variable displacement motor/pumps in hybrid vehicles is well known in the art. See, for example, U.S. Pat. No. 4,242,922.
When the driver of the vehicle depresses the accelerator pedal, the accumulator discharges into the reversible hydraulic driving motor or motors operating as such as to propel the vehicle. When the driver applies the vehicle brakes, the reversible hydraulic motor or motors operate as a pump run by the rotation of the moving vehicles wheels and, therefore, serve to recover part of the kinetic or potential energy of the moving vehicle in order to recharge the accumulator.
For highway driving conditions, a second mode of operation may be selected wherein the transmission connects the engine to the vehicle drive wheels and the engine continuously drives the transmission in a conventional manner. In this mode, the reversible hydraulic motor/pumps are stopped.
Such systems provide reduced fuel consumption, noise and atmospheric pollution under city traffic conditions as a result of energy recovery during brake application and as a result of intermittent operation of the engine. In addition, in low speed range of city mode the driver uses only an accelerator and a brake without the need to shift gears since the variable displacement hydraulic motor/pump provides continuously variable power transmission. Finally, the fuel consumption of the vehicle on highways corresponds to that of a conventional vehicle since the engine is coupled to the drive wheels in a conventional manner.
Shifting the transmission from the city mode to the highway mode typically involves decoupling the engine from the reversible variable displacement pump/motor and coupling the motor directly to the drive wheels. Conversely, shifting from highway mode to city mode typically involves decoupling the engine from the drive wheels and coupling the engine to the reversible variable displacement pump/motor.
A smooth transition between city and highway modes and different gear ratios within city mode is often sought so as to minimize jolting transmission components and to provide a more comfortable ride for the occupants of the vehicle. Torque dithering of the engine output during shifting has been used in the past to achieve smoother shifting. Torque dithering generally includes varying the torque output of the motor about a desired torque value so as to avoid issues such as gear tooth butting and/or jerky shifts. For example, it is known to modulate the torque output of an engine by controlling its fuel supply to achieve a desired torque output.